In such cables, the optical fibres are associated with a generally cylindrical elongate support structure which bears tractive forces. The support structure is generally made of metal or of a plastics substance such as polyethylene which may optionally be reinforced with a central steel wire. The support structure has helical grooves around its periphery, which may be of constant-pitch or of periodically-reversing pitch and the fibres are deposited in the grooves.
The cross-section of the grooves is larger than that of the fibres to allow them the possibility of some play and they are closed by a casing or tape which covers the support structure, the space left free in the grooves being filled with a viscous compound. These grooves serve to even out the stresses exerted on the fibres during the fibre winding operations, and during use of the resulting cable.
The assembly formed by the support structure and by the optical fibres is covered with an outer sheath, possibly with mechanical reinforcement and, if need be, with one or more intermediate layers of substance which acts as a shock absorber and of strong wire which acts as a protection against radial compression forces.
The mobility of the fibres in the grooves is satisfactory for bending stresses but not satisfactory for tractive stresses which elongate the support structure, in particular when these stresses are close to the breaking point of the support structure or when they are due to large amplitude thermal cycling.
Preferred embodiments of the present invention provide optical fibres whose length is sufficiently longer than that of the support structure for the tractive forces not to be transmitted to them as long as the support structure does not reach its breaking elongation limit.